An investigation of transformation strengthening in precipitation-hardened Fe-Ni austenite
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I. INTRODUCTION A reverse martensitic phase transformation influences the properties of austenite in a number of ferrous alloys. It has been established that a reverted austenite is appreciably stronger than an annealed austenite due principally to an increased dislocation d e n s i t y and to the f o r m a t i o n of a fine s u b s t r u c t u r e 1-8 r e s e m b l i n g the s t r u c t u r e of m a r t e n s i t e . The s t r e n g t h e n i n g achieved in an a n s t e n i t e through r e v e r s e m a r t e n s i t i c t r a n s f o r m a t i o n has b e e n r e f e r r e d to as " t r a n s formation strengthening." To t r a n s f o r m a t i o n s t r e n g t h e n an a n s t e n i t i c alloy one m u s t b e g i n with an alloy c o m p o s i t i o n such that the m a r t e n s i t e s t a r t t e m p e r a t u r e (Ms) is between r o o m t e m p e r a t u r e and liquid n i t r o g e n t e m p e r a t u r e (- 196~ The alloy may then be t r a n s f o r m e d to m a r t e n s i l e (at least in part) by cooling in liquid n i t r o g e n . The m a r t e n s i t i c alloy is then r e v e r t e d to a u s t e n i t e by heating to above the a n s t e n i t e f i n i s h t e m p e r a t u r e (Af) at a r a t e r a p i d enough to e n s u r e that the r e v e r s i o n r e a c t i o n p r o c e e d s through a r e v e r s e s h e a r m e c h a n i s m r a t h e r than through a diffusional n u c l e a t i o n and growth p r o c e s s . When the r e v e r s i o n r e a c t i o n is complete the alloy is cooled to r o o m t e m p e r a t u r e . F o r the t r a n s f o r m a t i o n s t r e n g t h e n i n g to have a b e n e f i c i a l effect on r o o m t e m p e r a t u r e p r o p e r t i e s the r e v e r t e d a u s t e n ite m u s t , of c o u r s e , be m e c h a n i c a l l y stable with r e S. JIN is Research Scientist, Bell Laboratories, Murray Hill, NJ 07974, 3. W. MORRIS, Jr. and G. THOMASare Professors of Metallurgy and Principal Investigators, Department of Materials Science and Engineering,and the Materials and Molecular Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, Y. L. CHEN is Professor, National Tsing Hua University, Department of Materials Science and Engineering,Hsinchu, Taiwan, and R. I. JAFFEE is Technical Manager, Materials, Fossil Fuel and Advance Systems Division,Electric Power Research Institute, Palo Alto, CA 94304. Manuscript submitted July 29, 1977. METALLURGICALTRANSACTIONSA
spect to s t r e s s - i n d u c e d t r a n s f o r m a t i o n to m a r t e n s i t e at loads below the m a t r i x yield s t r e n g t h . U s i n g this p r o c e d u r e K r a u s s and Cohen s d e m o n s t r a t e d the t r a n s f o r m a t i o n s t r e n g t h e n i n g of F e (30-34)Ni a l l o y s , obtaining a n i n c r e a s e in yield s t r e n g t h f r o m ~30 ksi (210 MPa) to ~60 k s i (420 MPa) a f t e r a V -7" a ' ~ V r e v e r s i o n cycle. In f u r t h e r work Koppenaal7 obtained a yield s t r e n g t h i n c r e a s e f r o m ~40 ksi (270
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